Capacitively coupled power supplies, often referred to as “capacitive dropper” or “cap dropper” power supplies, employ a capacitive element, operative as a current limiting element, placed in series with the incoming AC voltage where its effective impedance at the AC input's frequency serves to reduce, by virtue of the current flowing through that impedance, the voltage presented to an immediately following rectifier element. The resultant rectified DC current may be filtered by a filter capacitor, and a variety of feedback and control means have been employed to realize a regulated DC voltage at said filter capacitor.
In a basic cap dropper power supply design, the AC current from the current-limiting capacitor is rectified and conveyed to one terminal of a filter capacitor having its other terminal connected to a suitable common potential, such as GND, allowing a voltage to be developed on the filter capacitor. Voltage regulation is achieved by providing a passive shunt means, such as a Zener diode or an integrated shunt regulator, connected to limit the maximum voltage that can develop.
The basic design provides for a moderate degree of regulation to be achieved for a range of external loads. However, any of the incoming power that is not used by the external load must be dissipated directly in the shunt sub-circuit.
To achieve higher operational efficiency, a saturated switch element replaces the passive shunt means. Conventional feedback control drives actuation of this shunt switch element, actuating it whenever the voltage on the filter capacitor reaches or exceeds its set point. When the shunt switch element is ON, the voltage across it will be very near zero such that current flow into it from the output capacitor will be prevented by a blocking rectifier element.
One variant of the above method uses high-frequency switching to control the frequency of the ON/OFF events of the shunt switch element; this variant also incorporates a series diode or other switching element disposed between the output capacitor and the shunt switch, to avoid discharging the capacitor when the shunt switch is closed. The use of high frequency switching control in such implementations reduces the voltage ripple magnitude present at the filter capacitor. Further, the high-frequency nature of the ripple allows a relatively smaller capacitor value to achieve adequate filtering, but the higher switching frequencies also lead to higher EMI.